Expression of isolated DNA sequences in a plant host is dependent upon the presence of operably linked regulatory elements that are functional within the plant host. Choice of the regulatory sequences will determine when and where within the organism the isolated DNA sequence is expressed. Where continuous expression is desired in all cells of a plant throughout development, constitutive promoters are utilized. In contrast, where gene expression in response to a stimulus is desired, inducible promoters are the regulatory element of choice. Where expression in particular tissues or organs are desired, sometimes at specific stages of development, tissue-preferred promoters and/or terminators are used. That is, these regulatory elements can drive expression in specific tissues or organs, at specific stages. Additional regulatory sequences upstream and/or downstream from the core sequences can be included in expression cassettes of transformation vectors to bring about varying levels of expression of isolated nucleotide sequences in a transgenic plant.
Seed development involves embryogenesis and maturation events as well as physiological adaptation processes that occur within the seed to insure progeny survival. Developing plant seeds accumulate and store carbohydrate, lipid, and protein that are subsequently used during germination. Generally, the expression patterns of seed proteins are highly regulated. This regulation includes spatial and temporal regulation during seed development. A variety of proteins accumulate and decay during embryogenesis and seed development and provide an excellent system for investigating different aspects of gene regulation as well as for providing regulatory sequences for use in genetic manipulation of plants.
As the field of plant bioengineering develops, and more genes become accessible, a greater need exists for transforming with multiple genes. These multiple exogenous genes typically need to be controlled by separate regulatory sequences. Some genes should be regulated constitutively whereas other genes should be expressed at certain developmental stages or location in the transgenic organism. Accordingly, a variety of regulatory sequences having diverse effects is needed.
Another reason why diverse regulatory sequences are needed is that undesirable biochemical interactions result from using the same regulatory sequence to control more than one gene. For example, transformation with multiple copies of a regulatory element may cause homologous recombination between two or more expression systems, formation of hairpin loops caused from two copies of the same promoter or enhancer in opposite orientation in close proximity, competition between identical expression systems for binding to common promoter-specific regulatory factors, and inappropriate expression levels of an exogenous gene due to trans effects of a second promoter or enhancer.
In view of these considerations, a goal in this field has been the detection and characterization of new regulatory sequences for transgenic control of DNA constructs.
Isolation and characterization of seed-preferred promoters and terminators that can serve as regulatory elements for expression of isolated nucleotide sequences of interest in a seed-preferred manner are needed for improving seed traits in plants.